Pluto System After New Horizons 2019 (LPI Contrib. No. 2133) 7022.pdf Future Spacecraft Missions to the Pluto System. J. D. Hofgartner*1, B. J. Buratti1, M. W. Buie2, V. J. Bray3, E. Lellouch4, *[email protected], 1NASA Jet Propulsion Laboratory, California Institute of Technology, 2Southwest Research Institute, 3University of Arizona, 4Observatoire de Paris, Meudon. Introduction: The NASA New Horizons mission [13]. The system has diverse, unique, and ongoing revealed Pluto, Charon and their satellite system to be processes, all of which are valuable characteristics for far more active and complex than was expected for understanding the workings of solar systems. small icy worlds in the distant outer solar system [1-6]. Thus, further observation and exploration of the These discoveries indicate that further observation and Pluto-Charon system is valuable for all three of exploration of the Pluto system offers unique planetary science’s crosscutting themes. opportunities for fundamental advances in planetary Science Goals: The Pluto-Charon system offers a science. The Planetary Science Decadal Survey wide breadth of science questions covering many identified three crosscutting themes of particular disciplinary and multidisciplinary topics. We divide the importance in planetary science: building new worlds major science goals for future observations and (formation), planetary habitats (habitability) and spacecraft missions into six groups. workings of solar systems (workings) [7]; the Pluto Geology. Pluto displays diverse, unique, and active system offers important insights into all three of these surface processes and understanding these processes, themes. their interplay, and history should be a focus of future Formation. The Pluto-Charon binary is the only investigations. Other important geology questions known fully tidally evolved binary pair in the solar include whether liquids at the surface or near-surface system and was likely formed by a giant impact [5]. have influenced Pluto’s geology and whether Pluto has Thus, it is an important record of giant impacts and the stratigraphy in its crust, glaciers, and volatile deposits formation of binary systems. By virtue of its formation that are records of its geologic and climatic history. in the Kuiper belt, the Pluto system also records the Geophysics. Whether or not Pluto is a current ocean physical and chemical properties of the Kuiper belt. It world, and Charon a former ocean world, are key may also record important clues about the dynamical questions for understanding their evolution and rearrangement of the solar system [8]. habitability. Several important questions focus on Habitability. Organic matter is a crucial ingredient Sputnik Planitia including how and when the basin of a habitable environment [7] and Pluto’s Cthulhu formed, its total depth, and whether other similar basins Regio as well as other low albedo, equatorial maculae exist in the unseen southern hemisphere. The Pluto- are a large reservoir of processed organic matter Charon binary is in a tidal end-state and understanding (tholins) [3]. Pluto’s tholins are the most their mutual tidal evolution is an important goal for spectroscopically accessible complex organics in the understanding tides in general. solar system in the sense that they are the largest known Atmosphere and Climate. Hazes with spectacular deposits aside from those on Titan (and Earth) but are layering and continuity over hundreds of kilometers not obscured by an opaque atmosphere. Pluto is also a were discovered in Pluto’s atmosphere by New possible ocean world [9,10] with evidence for Horizons [4], generating many new questions. The geologically recent cryovolcanism [2] and Charon composition and structure of the haze molecules, their likely harbored an ocean in its past [11]. influence on the thermal structure of the atmosphere and Workings. Pluto is astoundingly active given the relationship with tholins on the surface, as well as what limited endogenic and solar power available for controls their layering, are all interesting new questions. geologic processes. Convective resurfacing, glacial Pluto’s climate likely varies over seasonal and mega- flow, and sublimation/condensation are all presently seasonal timescales, possibly including extreme ongoing [2,12]. Convection of Pluto’s 1000 km wide, excursions in pressure and temperature [14]. These nitrogen-rich Sputnik Planitia is a geologic process variations and how they affect the surface and unique to Pluto’s surface; it is the only known example atmosphere are not well understood. It is clear that where this very dynamic resurfacing process can be Pluto’s atmosphere and surface are intimately linked studied [12]. In contrast, glacial flow is a familiar and cannot be studied in isolation, future observations process on Earth and Mars, but the glaciers on Pluto are and missions must take this coupling into account. composed of entirely different volatiles, offering an Charon. Charon has extreme topographic variations excellent opportunity for comparing how composition with mountain tops and valley bottoms differing by > 20 and planetary conditions affect glacial processes [2]. km [15]. It has a roughly north-south hemispherical The Pluto system also exhibits volatile exchange dichotomy between rugged highlands and smooth plains between Pluto and Charon, resulting in a tholin deposit [2]. The formation of Charon’s topography and on Charon that is unique from all other icy satellites hemispherical dichotomy and their relation to a Pluto System After New Horizons 2019 (LPI Contrib. No. 2133) 7022.pdf subsurface ocean and cyrovolcanism are all important however, a spacecraft mission is necessary to address questions for future investigations. Other interesting the majority of the science. An orbiter mission is the questions focus on Charon’s Mordor Macula, which is likely best next step and would be competitive with unique among icy satellites, including its composition, other priority missions in planetary science. depth, and whether an analogous feature exists over Charon’s south pole [13]. References: [1] Stern+ (2015) Science 350, Small Satellites. The small satellites: Styx, Nix, aad1815. [2] Moore+ (2016) Science 351, 1284. [3] Kerberos, and Hydra, likely resulted from the binary Grundy+ (2016) Science 351, aad9189. [4] Gladstone+ forming impact and may record key clues about giant (2016) Science 351, aad8866. [5] Weaver+ (2016) impacts [5]; deciphering these clues is an important Science 351, aae0030. [6] Bagenal+ (2016) Science topic for future observations and missions. Their chaotic 351, aad9045. [7] National Research Council (2011) rotation and relationship to dynamical resonances are Vision and Voyages for Planetary Science in the Decade also interesting questions [5]. 2013-2022. The National Academies Press. [8] Exploration. New Horizons imaged ~80% of the Tsiganis+ (2005) Nature 435 459. [9] Keane+ (2016) surfaces of Pluto and Charon, but only ~40% at high- Nature 540, 90. [10] Nimmo+ (2016) Nature 540, 94. resolution, leaving full hemispheres yet to be explored [11] Beyer+ (2017) Icarus 287, 161. [12] McKinnon+ in detail. Other icy worlds in the solar system are known (2016) Nature 534, 82. [13] Grundy+ (2016) Nature to have dramatic hemispherical differences [16]. Basic 539, 65. [14] Stern+ (2017) Icarus 287, 47. [15] reconnaissance of the unexplored regions of the Pluto Schenk+ (2018) Icarus 315, 124. [16] Buratti+ (2017) system is an important goal for the future. Icarus 287, 207. [17] Hofgartner+ (2018) Icarus 302, Orbiter Mission: The nominal planetary 273. exploration track begins with Earth-based characterization, followed by flyby reconnaissance, then orbital observations, lander in situ investigations, mobile in situ elements such as rovers, and concluding with sample return. The Pluto system was characterized by many decades of observations prior to the New Horizons flyby exploration. Continued observations with Earth-based telescopes could address some of the outstanding questions for the Pluto system but the majority will require a spacecraft mission. Following the nominal exploration track, an orbiter mission is likely the best next step and indeed, with the appropriate instruments and capabilities, could address most of the science goals described above. The science goals for the Pluto system indicate that an orbiter mission is the likely best next mission, however, many aspects of such a mission require further study. We recommend that concepts for future orbiter missions include at a minimum the following key capabilities: (1) global observation of all surfaces in the Pluto system, including surfaces in polar night, (2) sampling of Pluto’s atmosphere and hazes, (3) close flybys of the four small satellites, primarily to measure their masses, and (4) observation of changes of Pluto’s atmosphere and surface over multiple timescales via comparison between the New Horizons flyby and orbiter data sets, and within the orbiter dataset [17]. Trade studies of the scientifically most compelling instruments are an important avenue for future work. In summary, the Pluto system is scientifically compelling with a diverse array of new questions following the New Horizons exploration. Future observations of the system with Earth-based telescopes and near-Earth space telescopes (such as JWST) will continue to refine our understanding of the system, .